The proposed project consists of a continuation of studies by freeze-fracture and ultrastructural methods of myelinated nerve fibers intended to generate new data about structural specializations in the nodal and paranodal axolemma. Both peripheral and central axons will be examined during development and degeneration in order to determine the sequence in which membrane specializations of both the axolemma and associated glial and Schwann cell membranes appear and disappear. Specific labeling of sodium channels will also be attempted in order to more precisely define their location and concentration. Observations will also be made on the reappearance of structural specializations in regenerating fibers in order to determine whether the same sequence of events is followed as in normal embryological development. These studies will be extended to include initial segments of axons in order to determine whether the structural specializations here correspond to those at the node of Ranvier. Comparisons will be made between nerves from mammals and amphibians.